The electrochemical carbon dioxide (CO₂) reduction to ethylene (C₂H₄) and ethanol (C₂H₅OH) using renewable energy is a viable method for the production of these commercially vital chemicals. Copper (Cu) based materials are by far the most promising electrocatalysts for this purpose. However, the formation of ethanol using Cu and its oxides is generally less favored as compared to that of ethylene. Here, we demonstrate that the selectivity of CO₂ reduction towards ethanol could be systematically tuned by introducing a co-catalyst to generate an in situ source of mobile CO reactant. Cu-based oxides with different amount of Zn dopants (Cu, Cu₁₀Zn, Cu₄Zn and Cu₂Zn) were prepared via electrodeposition and used as catalysts under ambient pressure in aqueous 0.1 M KHCO₃ electrolyte. By varying the concentration of Zn in the bimetallic catalysts, we found that the selectivity of ethanol versus ethylene formation, defined by the ratio of their faradaic efficiencies (FE_ethanol/FE_ethylene), could be tuned by a factor of up to ~12.5. Ethanol selectivity was maximized on Cu₄Zn at -1.05 V vs. RHE, with a remarkable faradaic efficiency and current density of 29.1% and -8.2 mA/cm² respectively. The Cu₄Zn catalyst was also catalytically stable towards ethanol formation for at least 5 hrs.

Operando Raman spectroscopy revealed that the as-deposited oxides were reduced to the metallic Cu/Zn during CO₂ reduction, after which only signals belonging to CO adsorbed on Cu sites were recorded. This indicated that the reduction of CO₂ might preferably occur on metallic sites rather than on metal oxides. The importance of Zn as a CO producing site was also demonstrated by performing CO₂ reduction on Cu-Ni and Cu-Ag bimetallic catalysts. Thus, the mechanism of ethanol formation was proposed as following: Incoming CO₂ molecules could first bind to either Cu or Zn sites, and be reduced to CO. On the Cu sites, CO could be reduced further to CHO or CH_x (x=1,2,3) intermediates, while CO adsorbs weakly on Zn sites and is likely to desorb. The desorbed CO could diffuse and spill over onto the Cu sites. The spilled-over CO may then insert itself into the bond between the Cu surface and *CH₂, to form ^*COCH₂. Further reduction of ^*COCH₂ will produce acetaldehyde and finally ethanol.

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